Field of the Invention
The present invention relates to an improved antenna for transmitting and receiving radiation, and more particularly to a simplified and highly efficient antenna having a physical length which is short relative to the wavelength of the radiation and which is broadband. It also relates to automotive and other mobile system use of a single short antenna which can be coupled (1) in one way as the transmitting-receiving antenna for a Citizens Band radio or (2) in another way as a electrically-short efficient antenna to receive signals on the bands from AM to and including FM.
In my previous patent U.S. Pat. No. 4,675,691, incorporated herein by reference, I described an arrangement of electrodes which showed how an electrically-short antenna can be constructed by using an electrostatic capacitor such a split-cylindrical capacitor, as the radiating member of a resonant circuit. In this patent, the conductors forming the capacitors are concave surfaces.
As described herein, a short length antenna is defined as one which has a length equal to or less than one quarter of a wavelength (.lambda./4) of its resonant frequency. Usually, such short antennas typically exhibited a high Q or a rather sharp tuning peak.
In the present invention, we describe how we have subsequently found the new forms of capacitors can be made to operate as broad-band, efficient antennas.
To understand the previous theoretical appraisals of these type of antennas we refer to the following references, incorporated herein by reference.
Kraus, John D., "Antennas" 2nd Ed., McGraw-Hill, N.Y., 1988, especially pp. 711-714.
Hansen, R. C., "Fundamental Limitations of Antennas," Proc. IEEE, 69, 170-182, February, 1981.
Wheeler, H. A., "Fundamental limitations of small antennas," Proc. IRE, vol 35 pp. 1479-1484, Dec. 1947.
Ramo, Simon, and J. R. Whinnery, "Fields and Waves in Modern Radio" John Wiley & Sons, Inc , New York, N.Y., 1944, pp 432 and 458-459.
Professor Kraus, widely recognized as one of the foremost authorities on antennas, devotes a section of his recent book to the properties of electrically-short antennas. He relies on the work of R. C. Hansen and Wheeler, to conclude that the radiation resistance decreases with increasing wavelength, and that therefore no electrically small, efficient antenna is possible. This result is understandable since the treatment of antenna radiation for short antenna structures have assumed that the radiation takes place by means of dipole radiation formed by wires connected to the antenna structures.